This invention relates to a method of manufacturing a semiconductor article that can suitably be used for producing a semiconductor device such as a semiconductor integrated circuit, a solar cell, a semiconductor laser device or a light emitting diode. More particularly, it relates to a method of manufacturing a semiconductor article comprising a step of transferring a semiconductor layer onto a substrate.

Semiconductor articles are popular in terms of semiconductor wafers, semiconductor substrates and various semiconductor devices and include those adapted for producing semiconductor devices by utilizing the semiconductor region thereof and those used as preforms for producing semiconductor devices.

Some semiconductor articles of the type under consideration comprise a semiconductor layer arranged on an insulator.

The technology of forming a single crystal silicon semiconductor layer on an insulator is referred to as silicon on insulator (SOI) technology, which is widely known. Various researches have been done to exploit the remarkable advantages of SOI that cannot be achieved by using bulk Si substrates that are used for producing ordinary Si integrated circuits. The advantages of the SOI technology include:

• 1. the ease of dielectric isolation that allows an enhanced degree of integration;
• 2. the excellent resistivity against radiation;
• 3. a reduced floating capacitance that allows a high device operation speed;
• 4. the omission of the well forming step;
• 5. the effect of latch up prevention; and
• 6. the possibility of producing fully depleted field effect transistors using the thin film technology. The advantages of the SOI technology are thoroughly discussed in Special Issue: "Single-crystal silicon on non-single-crystal insulators"; edited by G. W. Cullen, Journal of Crystal Growth, volume 63, No. 3, pp. 429-590 (1983).

In recent years, a number of reports have been published on the SOI technology for providing substrates that can realize high speed operation and low power consumption for MOSFETs (IEEE SOI conference 1994). The process of manufacturing a semiconductor device can be significantly curtailed by using the SOI structure if compared with the corresponding process of manufacturing a device on a bulk Si wafer because of the implementation of a very simplified device isolation step. Thus, the use of the SOI technology can provide a significant cost reduction in manufacturing a semiconductor device particularly in terms of the wafer cost and the process cost if viewed from the conventional technology of manufacturing a MOSFET or an IC on a bulk Si substrate, to say nothing of the remarkable performance of such a semiconductor device.

Fully depleted MOSFETs are very promising for achieving high speed operation and low power consumption if provided with improved drive power. Generally speaking, the threshold voltage (Vth) of a MOSFET is determined as a function of the impurity concentration of its channel section but, in the case of a fully depleted (FD) MOSFET, the characteristics of the depletion layer is influenced by the SOI film thickness. Therefore, the SOI film thickness has to be rigorously controlled in order to improve the yield of manufacturing LSIs.

Meanwhile, a device formed on a compound semiconductor shows a remarkable level of performance that cannot be expected from silicon particularly in terms of high speed operation and light emission. Such devices are currently formed by means of epitaxial growth on a compound semiconductor substrate that may be made of GaAs or a similar compound. However, a compound semiconductor substrate is costly and mechanically not very strong so that it is not adapted to produce a large wafer.

Thus, efforts have been made to form a compound substrate by hetero-epitaxial growth on an Si wafer that is inexpensive, mechanically strong and good for producing a large wafer.

Researches on forming SOI substrates became remarkable in the 1970s. Initially, attention was paid to the technique of producing single crystal silicon by epitaxial growth on a sapphire substrate (SOS: silicon on sapphire), that of producing an SOI structure through full isolation by porous oxidized silicon (FIPOS) and the oxygen ion implantation technique. The FIPOS method comprises steps of forming an islanded N-type Si layer on a P-type single crystal Si substrate by proton/ion implantation (Imai et al., J.Crystal Growth, Vol. 63,547 (1983)) or by epitaxial growth and patterning, transforming only the P-type Si substrate into a porous substrate by anodization in an HF solution, shielding the Si islands from the surface, and then subjecting the N-type Si islands to dielectric isolation by accelerated oxidation. This technique is, however, accompanied by a problem that the isolated Si region is defined before the process of producing devices to restrict the freedom of device design.

The oxygen ion implantation method is also referred to the SIMOX method, which was proposed by K. Izumi for the first time. With this technique, oxygen ions are implanted into an Si wafer to a concentration level of 10¹⁷ to 10¹⁸/cm² and then the latter is annealed at high temperature of about 1,320°C in an argon/oxygen atmosphere. As a result, the implanted oxygen ions are chemically combined with Si atoms to produce a silicon oxide layer that is centered at a depth corresponding to the projection range (Rp) of the implanted ions. Under this condition, an upper portion of the Si oxide layer that is turned into an amorphous state by the oxygen ion implantation is recrystallized to produce a single crystal Si layer. While the surface Si layer used to show a defect rate as high as 10⁵/cm², a recent technological development has made it possible to reduce the defect rate down to about 10²/cm² by selecting a rate of oxygen implantation of about 4×10¹⁷/cm². However, the allowable range of energy infusion and that of ion implantation are limited if the film quality of the Si oxide layer and the crystallinity of the surface Si layer are to be held to respective desired levels and hence the film thickness of the surface Si layer and that of the buried Si oxide (BOX; buried oxide) layer are allowed to take only limited values. In other words, a process of sacrifice oxidation or epitaxial growth is indispensable to realize a surface Si layer having a desired film thickness. Such a process by turn gives rise to a problem of uneven film thickness due to the intrinsic adverse effect of the process.

There have been reports saying that SIMOX can produce defective Si oxide regions in the Si oxide layer that are referred to as pipes. One of the possible causes of the phenomenon may be foreign objects such as dust introduced into the layer at the time of ion implantation. The device produced in a pipe region can show degraded characteristics due to the leak current between the active layer and the underlying substrate.

The SIMOX technique involves the use of a large volume of ions that is by far greater than the volume used in the ordinary semiconductor process and hence the ion implantation process may take a long time if a specifically designed apparatus is used for it. Since the ion implantation process is performed typically by means of raster scan of an ion beam showing a predetermined flow rate of electric current or by spreading an ion beam, a long time may be required for processing a large wafer. Additionally, when a large wafer is processed at high temperature, the slip problem due to an uneven temperature distribution within the wafer can become very serious. Since the SIMOX process requires the use of extraordinary high temperature that is as high as 1,320°C which is not observed in the ordinary Si semiconductor process, the problem of uneven temperature distribution will become more serious if a large wafer has to be prepared unless a highly effective apparatus is not realized.

Beside the above described known techniques of forming SIO, a technique of bonding a single crystal Si substrate to another single crystal Si substrate that has been thermally oxized to produce an SOI structure has been proposed recently. This method requires the use of an active layer having an even thickness for the devices to be formed on it. More specifically, a single crystal Si substrate that is as thick as several hundreds micrometers has to be made as thin as several micrometers or less. Three techniques have been known for thinning a single crystal Si layer that include;

• (1) polishing,
• (2) local plasma etching and
• (3) selective etching.

It is difficult to achieve an even film thickness by means of the polishing technique of (1) above. Particularly, the mean deviation in the film thickness can be as large as tens of several percent to make the technique unfeasible when the film is thinned to an order of sub- micrometer. This problem will become more remarkable for wafers having a large diameter.

The technique of (2) is typically used in combination with that of (1). More specifically, the film is thinned by means of the technique of (1) to about 1 to 3µm and the distribution of film thickness is determined by observing the film thickness at a number of points. Then, the film is subjected to an etching operation where the film is scanned with plasma of SF₆ particles having a diameter of several millimeters, correcting the distribution of film thickness, until a desired film thickness is obtained. There has been a report that the distribution of film thickness can be confined within about ±10nm or less by means of this technique. However, this process is accompanied by a drawback that, if foreign objects are present on the substrate in the form of particles during the plasma etching, they operate as so many etching masks to produce projections on the substrate when the etching operation is over.

Additionally, since the substrate shows a coarse surface immediately after the etching operation, a touch- polishing operation has to be conducted on the surface after the end of the plasma etching and the operation is controlled only in terms of its duration. Then, again the problem of deviations in the film thickness due to polishing arises. Still additionally, a polishing agent typically containing colloidal silica is used for the polishing operation and hence the layer for making an active layer is directly scraped by the polishing agent so that a crushed and/or distorted layer may be produced. The throughput of the process can be significantly reduced when large wafers are treated because the duration of the plasma etching operation is prolonged as a function of the surface area of the wafer being processed.

The technique of (3) involves the use of a film configuration for the substrate to be thinned that comprises one or more than one film layers adapted to selective etching. For example, assume that a P⁺-Si thin layer containing boron by more than 10¹⁹/cm³ and a P-type Si thin layer are made to grow sequentially on a P-type substrate by means of epitaxial growth to produce a first substrate, which is then bonded to a second substrate with an insulation layer interposed therebetween, the insulation layer being typically an oxide film, and that the rear surface of the first substrate is made sufficiently thin in advance by scraping and polishing. Subsequently, the P⁺-layer is made to become exposed by selectively etching the overlying P-type layer and then the P-type substrate is made to become exposed by selectively etching the P⁺-layer to produce an SOI structure. This technique is discussed in detail in a report by Maszara (W. P. Maszara, J. Electrochem. Soc., Vol. 138,341 (1991)).

While the selective etching technique is effective for producing a thin film with an even film thickness, it is accompanied by the drawbacks as identified below.

• -- The selective etching ratio is not satisfactory and will be as low as 10² at most.
• -- A touch-polishing operation is required to smooth the surface after the etching operation because of the coarse surface produced by the etching operation. Therefore, the film thickness can lose the uniformity as it is reduced by polishing. Particularly, while the polishing operation is controlled by the duration of the operation, it is difficult to rigorously control the operation because the polishing rate can vary significantly from time to time. Thus, this problem becomes unnegligible when forming an extremely thin SOI layer that is as thin as 100nm.
• -- The produced SOI layer can show a poor crystallinity due to the use of a film forming technique that involve ion implantation and epitaxial or hetero-epitaxial growth on an Si layer that is densely doped with B. Additionally, the bonded surface of the substrate may show a degree of smoothness that is inferior relative to that of a conventional Si wafer (C. Harendt, et al., J. Elect. Mater. Vol. 20,267 (1991), H. Baumgart, et al., Extended Abstract of ECS 1st International Symposium of Wafer Bonding, pp-733 (1991), C. E. Hunt, Extended Abstract of ECS 1st International Symposium of Wafer Bonding, pp-696 (1991)). Still additionally, there is a problem that the selectivity of the selective etching technique heavily depends on the concentration difference among the impurities such as boron contained in the substrate and the steepness of the concentration profile of the impurities along the depth of the substrate. Therefore, if the bonding anneal is conducted at high temperature to improve the bonding strength of the layers and the epitaxial growth is carried out also at high temperature to enhance the crystallinity of the SOI layer, the concentration profile of the impurities along the depth becomes flattened to reduce the selectivity of the etching operation. Simply stated, the improvement of the etching selectivity and hence that of the crystallinity and the improvement of the bonding strength are conflicting requirements that cannot be met at the same time.

Under these circumstances, the inventors of the present invention proposed a novel method of manufacturing a semiconductor article in Japanese Patent Application Laid-Open No. 5-21338. According to the invention, the proposed method is characterized by comprising steps of forming an article by arranging a nonporous single crystal semiconductor region on a porous single crystal semiconductor region, bonding the surface of a material carrying an insulating material thereon to the corresponding surface of said porous single crystal semiconductor region and subsequently removing said porous single crystal semiconductor region by etching.

T. Yonehara et al. who are the inventors of the present invention also reported a bonded SOI that is excellent in terms of even film thickness and crystallinity and adapted to batch processing (T. Yonehara et al., Appl. Phys. Lett. Vol. 64, 2108 (1994)). Now, the proposed method of manufacturing a bonded SOI will be summarily described below by referring to FIGS. 4A through 4C of the accompanying drawings.

The proposed method uses a porous layer 32 formed on a first Si substrate 31 as a layer to be selectively etched. After forming a nonporous single crystal Si layer 33 on the porous layer 32 by epitaxial growth, it is bonded to a second substrate 34 with an Si oxide layer 35 interposed therebetween (FIG. 4A). Then, the porous Si layer is made to become exposed over the entire surface area of the first substrate by scraping off the first substrate from the rear side (FIG. 4B). The exposed porous Si is then etched out by means of a selective etching solution typically containing KOH or HF+H₂O₂ (FIG. 4C). Since the selective etching ratio of the operation of etching the porous Si layer relative to the bulk Si layer (nonporous single crystal Si layer) can be made as high as hundreds of thousands with this technique, the nonporous single crystal Si layer formed on the porous layer in advance can be transferred onto the second substrate to produce an SOI substrate without reducing the thickness of the nonporous single crystal Si layer. Thus, the uniformity of the film thickness of the SOI substrate is determined during the epitaxial growth step. According to a report by Sato et al., since a CVD system adapted to an ordinary semiconductor process can be used for the epitaxial growth, a degree of uniformity of the film thickness as high as 100nm±2% can be realized. Additionally, the epitaxial Si layer shows an excellent crystallinity of about 3.5×10²/cm².

Since the selectivity of any conventional selective etching technique heavily depends on the concentration difference among the impurities contained in the substrate and the steepness of the concentration profile of the impurities along the depth of the substrate as described above, the temperature of the heat treatment (for bonding, epitaxial growth, oxidation and so on) is limited to as low as 800°C at most because the impurity concentration profile becomes flattened above that temperature limit. On the other hand, the etching rate of the proposed etching technique is mainly determined by the structural difference between the porous layer and the bulk layer so that the heat treatment is not subjected to such a rigorous limitation and temperature as high as 1,180°C can be used. It is known that a heat treatment process conducted after the bonding operation can remarkably improve the bonding strength between wafers and reduce the size and number of voids given rise to on the bonding interface. Additionally, with a selective etching operation relying the structural difference between the porous layer and the bulk layer, the uniformity of the film thickness is not adversely affected by fine particles that can be adhering to the porous Si layer.

However, a semiconductor substrate to be produced by way of a bonding process inevitably requires at least two wafers as starting materials, one of which is substantially wasted away in the course of polishing and etching to consume the limited natural resources almost for nothing. In other words, an SOI manufacturing process is required to realize low cost and economic feasibility in addition to an enhanced degree of process controllability and an improved uniformity of the film thickness.

Differently stated, the requirements of a process for manufacturing a high quality SOI substrate include an excellent reproducibility, an enhanced level of resource saving capability through the repeated use of a same wafer and low manufacturing cost.

Under these circumstances, the inventors of the present invention proposed in Japanese Patent Application Laid-Open No. 7-302889 a method of manufacturing a semiconductor substrate, with which a pair of substrates are bonded together and subsequently separated from each other through a porous layer arranged therebetween so that one of the substrates may be reused by removing the porous substance remaining on it. The disclosed method will now be summarily described below by referring to FIGS. 5A through 5C of the accompanying drawings.

It comprises steps of forming a porous layer 42 by transforming a surface layer of a first Si substrate 41 into a porous state, forming a single crystal Si layer 43 on the porous layer, bonding the single crystal Si layer to the main surface of a second Si substrate 44 with an insulation layer 45 interposed therebetween (FIG. 5A). It further comprises steps of separating the wafers bonded together with the porous layer arranged therebetween (FIG. 5B) and selectively removing the exposed porous Si layer on the surface of the second Si substrate to produce an SOI substrate (FIG. 5C). With this method, the first substrate 41 can be reused after removing the residual porous layer. The bonded wafers may be separated from each other typically by way of one of the following techniques;

• -- applying sufficiently strong tensile force or pressure onto a surface of the combined wafers along a direction perpendicular to the surface;
• -- applying wave energy in the form of an ultrasonic wave or the like to the combined wafers;
• -- causing the porous layer to be exposed at an end surface of the combined wafers, etching the porous Si layer to a certain extent and inserting the edge of a blade;
• -- causing the porous layer to be fully exposed at an end surface of the wafers, soaking the porous Si layer with liquid that may be water and causing the liquid to expand by entirely heating or cooling the combined wafers; and
• -- applying force to the first (or second) substrate along a direction parallel to the second (or first) substrate in order to destroy the porous Si layer.

The above listed techniques are based on the idea that, while the mechanical strength of the porous Si layer depends on the porosity of the layer, it is sufficiently lower than that of a bulk Si layer. As a rule of thumb, a porous Si layer having a porosity of 50% shows a mechanical strength about a half of that of a corresponding bulk Si layer. In short, when a pair of bonded wafers is subjected to compressive, tensile or shearing force, the porous Si layer will be destroyed to begin with. A porous layer showing a higher degree of porosity can be destroyed with less force.

However, in reality, efforts have been paid to reduce the porosity of the surface layer of the porous Si in order to realize an excellent epitaxial growth in terms of the quality of the device formed on the SOI substrate, while increasing the porosity of the inside of the porous Si for easy separation of the bonded wafers. Thus, as described in an example disclosed in Japanese Patent Application Laid-Open No. 7-302889, it has been a known practice to modify the porosity of the porous Si layer by controlling the electric current used in an anodization process.

On the other hand, the above identified Japanese Patent Application Laid-Open No. 7-302889 discloses that a substrate that has been used for preparing a semiconductor substrate can be reused for preparing another semiconductor substrate and hence it provides a very useful technique for reducing the cost of manufacturing semiconductor substrates.

More specifically, Japanese Patent Application Laid-Open No. 7-302889 discloses a technique that uses anodization for transforming a silicon substrate into a porous state. In a plant where a large number of silicon substrates have to be uniformly transformed into a porous state, the specific resistance of the substrate has to be rigorously controlled because an anodization process utilizes an anodic reaction of silicon. However, a silicon substrate showing a specified specific resistance is costly and hence the cost of manufacturing an SOI substrate can be significantly reduced if it can prepared disregarding its specific resistance.

In view of the above described circumstances, it is therefore an object of the present invention to provide a method of manufacturing a semiconductor article comprising a step of bonding a pair of substrates, wherein part of the substrates may be reused as raw material for manufacturing another semiconductor article.

The method of manufacturing a semiconductor article considered herein is a method of the kind, such as described in Patent Abstracts of Japan, vol. 96 No. 3, 29 March 1996 and Japanese Laid Open Patent Application JP-07-302889, comprising steps of:

• forming a high concentration impurity layer with an impurity concentration in the range of between 5.0x10¹⁶/cm³ and 5.0x10²⁰/cm³ at the surface of a silicon substrate, using as dopant an element for determining the conductivity type of said layer;
• forming a porous silicon layer in the surface of the silicon substrate by anodisation;
• preparing a first substrate by forming a non-porous semiconductor layer on said porous silicon layer;
• bonding said first substrate and a second substrate together to produce a multilayer structure with said non-porous semiconductor layer sandwiched between said silicon substrate and said second substrate;
• splitting said multilayer structure at said porous silicon layer; and
• removing the remanent portion of said porous silicon layer remaining on said non-porous semiconductor layer.

This method is, in accordance with the present invention, characterised in that:

• said step of forming said porous silicon layer by anodisation is performed to form said porous silicon layer to a depth exceeding the depth of said high concentration impurity layer.

It is possible to reuse the substrate obtained by removing the remanent portion of the porous layer remaining on the separated first substrate as material of another first substrate or of another second substrate for use in the above-mentioned method.

With a method of manufacturing a semiconductor article according to the invention, after forming the high concentration impurity region at the surface of the silicon substrate, a porous silicon layer is formed in a region including said impurity region. Thus, a desired value can be selected for the concentration of the element (impurity concentration as generally referred to in the field of the semiconductor industry) capable of controlling conductivity in the impurity region that provides the basis of forming a porous silicon layer. In other words, a relatively low cost silicon wafer or a regenerated wafer that does not have a specified resistivity can be used for various applications where a silicon substrate having a specified specific resistivity is used ("a regenerated wafer" as used herein refers to a wafer obtained by polishing and removing the surface layer of a monitor wafer used in an IC process or a wafer carrying thereon one or more than one defective devices to such an extent that it can be reused in an IC process). Therefore, with a method of manufacturing a semiconductor article according to the invention, a semiconductor article can be manufactured at a cost by far lower than the cost of manufacturing it by means of any known manufacturing method. Additionally, the region of the porous silicon layer other than the impurity region is structurally by far more fragile than the impurity region that is porous so that it can be destroyed very easily in the splitting process and the first and second substrates can be separated from each other easily and reliably. In other words, a fragile structure can be produced within the porous silicon layer without modifying the electric current in the course of the process of forming the porous silicon layer. Thus, a couple of porous silicon layers can be automatically and sequentially formed without changing the anodizing conditions so that the surface porous silicon layer may be used for forming a high quality epitaxial layer while the underlying porous silicon layer may effectively operate as separation layer.

Respective impurity regions can be formed on both the principal surface and the rear surface of the silicon substrate by means of a diffusion technique. With a silicon wafer carrying a diffusion region on both of its oppositely disposed surfaces, the electric resistance between the rear surface of the wafer and the + electrode can be reduced for forming a porous layer to a great advantage of causing a uniform current to flow along the surface of the wafer so that a porous silicon layer may be formed with a very uniform thickness. While a technique such as ion implantation or epitaxial growth may alternatively be used for forming a high concentration impurity region (particularly P⁺ layer), unlike a diffusion technique, these techniques are effective only for treating only one of the oppositely disposed surfaces of a substrate and hence cannot be used to produce a P⁺ layer of the rear surface of the substrate. Additionally, these techniques are costly if compared with a diffusion technique, although they can treat only one of the oppositely disposed surfaces of a substrate. Differently stated, a diffusion technique is advantageous in that it can treat both of the oppositely disposed surfaces of a substrate at relatively low cost.

Additionally, a method of manufacturing a semiconductor article according to the invention is effective in producing a highly crystalline single crystal semiconductor layer on a surface of a second substrate that may be made of an insulating substrate and, therefore, it operates excellently for manufacturing semiconductor articles in terms of productivity, uniformity, controllability and cost.

Finally, a semiconductor article manufactured by a method according to the invention is less costly and hence can effectively replace a costly SOS or SIMOX in the process of manufacturing a large scale integrated circuit having an SOI structure.

• FIGS. 1A, 1B, 1C, 1D and 1E are schematic cross sectional lateral views of a semiconductor article being manufactured by a method according to the invention, illustrating different manufacturing steps in a mode of carrying out the invention.
• FIGS. 2A, 2B, 2C, 2D and 2E are schematic cross sectional lateral views of a semiconductor article being manufactured by a method according to the invention, illustrating different manufacturing steps in another mode of carrying out the invention.
• FIG. 3 is a schematic cross sectional view of a furnace that can be used for forming a diffusion region for the purpose of the invention.
• FIGS. 4A, 4B and 4C are schematic cross sectional lateral views of a semiconductor article being manufactured by a first known method.
• FIGS. 5A, 5B and 5C are schematic cross sectional lateral views of a semiconductor article being manufactured by a second known method.

Now, the present invention will be described in greater detail in terms of preferred modes and different phases of carrying out the invention. However, it will be appreciated that the present invention is by no means limited thereto and covers any other modes of realizing the invention that can be used for the purpose of the invention.

For the purpose of the present invention, an element adapted to being diffused into a silicon substrate by means of a diffusion technique and capable of controlling the conduction type refers to an element that is popularly used with a technique of processing a semiconductor material and can be selected from the elements listed in Table 1 below. Elements Capable of Controlling the Conduction Type conduction type element n P, As, Sb p B

The diffusion technique to be used for the purpose of the present invention is preferably adapted to thermally diffuse an element capable of controlling the conduction type into a silicon substrate. Diffusion techniques that can be used for the purpose of the present invention include those listed in Table 2 below. Techniques for Diffusing an Element diffusion technique diffusion source furnace open pipe technique POCl₃, BN, PH₃ diffusion furnace application technique a piece of glass applied with the element diffusion furnace use of doped film doped oxide (CVDPSG) doped poly-Si diffusion furnace sealed pipe tecnique As (solid), BCl₃, PH₃ capsule diffusion furnace

For the purpose of the invention, a porous layer is formed within a diffusion region more easily when the diffusion region is a P-type diffusion region than when it is an N-type diffusion region. In view of this fact, techniques that can be used for diffusion B (boron) are listed in Table 3 below. Boron Diffusion Techniques gaseous source B₂H₆ liquid source BBr₃ solid source B₂O₃ solid phase-solid phase diffusion CVD film, BSG, spin-coated film

With any of the techniques listed in Table 3, the element supplied from the source is diffused into a silicon substrate by means of a heat treatment operation conducted in a "furnace".

For example, the diffusion technique using a spin-coated film is carried out in a manner as described below.

Firstly, a mixture of B₂O₃, an organic binder and a solvent is evenly applied onto a silicon substrate (wafer) by means of a spinner. The applied mixture is dried and baked to form a B₂O₃ film on the silicon substrate. Thereafter, the silicon substrate is placed in a furnace having a configuration as shown in FIG. 3 and subjected to a heat treatment process, where boron (B) atoms are diffused into the substrate. In FIG. 3, reference numeral 301 denotes the furnace and reference numeral 302 denotes a susceptor. The silicon substrate 100 is coated on one of the surfaces thereof with a B₂O₃ film. Thus, boron (B) atoms can be diffused into the silicon substrate by heating the substrate at 900 to 1,300°C in the furnace. Note that a diffusion region is formed not only on the surface that carries thereon the B₂O₃ film but also on the oppositely disposed surface of the substrate when another silicon substrate carrying thereon a B₂O₃ film is arranged within the furnace with the B₂O₃ film carrying surface located vis-a-vis the surface of the first substrate carrying no B₂O₃ film thereon because the B₂O₃ film of the second substrate operates as a source for supplying B₂O₃.

A diffusion layer is formed on both of the oppositely disposed surfaces of the silicon substrate is advantageous because it can reduce the contact resistance between the substrate and the HF solution that is used in the anodization process for forming a porous silicon layer in the diffusion region.

For the purpose of the invention, the concentration of the element contained in the diffusion region and capable of controlling the conduction type is generally between 5.0×10¹⁶/cm³ and 5.0×10²⁰/cm³, preferably between 1.0×10¹⁷/cm³ and 2.0×10²⁰/cm³ and most preferably between 5.0×10¹⁷/cm³ and 1.0×10²⁰/cm³ from the viewpoint of the process of producing porous silicon through transformation and the characteristics of the epitaxial film formed on the porous silicon layer.

For the purpose of the invention, the thickness of the diffusion region is controllable by controlling the temperature and the duration of the heat treatment process. It is generally between 10 nm (100Å) and 100 µm, preferably between 50 nm (500Å) and 50µm and most preferably between 500 nm (5,000Å) and 30 µm. However, it is not necessarily required to form a thick diffusion region because the process of transforming it into a porous state that follows the process of forming the diffusion region can easily spread beyond the diffusion region.

For the purpose of the present invention, basically any single crystal silicon substrate (silicon wafer) can be used for the silicon substrate on which a diffusion region is formed. However, from the viewpoint of manufacturing a semiconductor substrate at low cost, a silicon substrate having an unspecified electric resistance, a monitor wafer that has been used in an IC process or a regenerated wafer obtained by polishing the surface layer of a substrate carrying thereon one or more than one defective devices to such an extent that it may be reused for an IC process may advantageously be used for the purpose of the invention.

Porous Si was firstly discovered in 1956 by Uhlir et al. who were studying a process of electropolishing a semiconductor material (A. Uhlir, Bell Syst. Tech. J., Vol. 35,333 (1956)). Porous Si can be prepared through anodization of an Si substrate in an HF solution. Unagami reports as a result of his study on the dissolutive reaction of Si in an Si anodization process that the existence of holes is required for anodization of Si and the reaction proceeds in a manner as described below (T. Unagami, J. Electrochem. Soc., Vol. 127,476 (1980)). $${\text{Si + 2HF + (2-n)e}}^{\text{+}}{\text{→ SiF}}_{\text{2}}{\text{+ 2H}}^{\text{+}}{\text{+ ne}}^{\text{-}}$$$${\text{SiF}}_{\text{2}}{\text{+ 2HF → SiF}}_{\text{4}}{\text{+ H}}_{\text{2}}$$$${\text{SiF}}_{\text{4}}{\text{+ 2HF → H}}_{\text{2}}{\text{SiF}}_{\text{6}}$$or $${\text{Si + 4HF + (4-λ)e}}^{\text{+}}{\text{→ SiF}}_{\text{4}}{\text{+ 4H}}^{\text{+}}{\text{+ λe}}^{\text{-}}$$$${\text{SiF}}_{\text{4}}{\text{+ 2HF → H}}_{\text{2}}{\text{SiF}}_{\text{6}}$$where e⁺ and e^- represent respectively a hole and an electron and n and λ represent respective numbers of holes required for dissolving a single Si atom. The report says that porous Si is formed when the condition of n>2 or λ>4 is met.

Although a conclusion that can be drawn from the above is that P-type Si can be made porous under the existence of holes whereas N-type Si cannot be made porous, in reality, both N-type Si and P-type ^Si can be turned porous under certain conditions.

For the purpose of the invention, single crystal porous Si can be formed through anodization of a single crystal Si substrate typically in an HF solution. A porous Si layer shows a spongy structure where pores with a diameter between 10^-1 and 10nm are arranged with intervals between 10^-1 and 10nm. The density of porous Si can be made to vary between 2.1 and 0.6g/cm³ by varying the concentration of the HF solution between 50 and 20% and/or by varying the current density in contrast to the density of single crystal Si that is equal to 2.33g/cm³. In other words, the porosity of porous Si is variable. While porous Si can be made to show a density less than a half of that of single crystal Si, it maintains the properties as single crystal Si so that a single crystal Si layer can be formed by epitaxial growth on a porous Si layer.

A porous Si layer has a density that is less than the density of a single crystal Si layer because it contains a large number of voids in the inside. Consequently, a porous Si layer shows a dramatically large surface area relative to the volume it occupies. This means that a porous Si layer can be etched at a rate by far greater than the rate at which an ordinary single crystal Si layer is normally etched.

While porous Si shows a mechanical strength that varies depending on its porosity, it is presumably lower than that of bulk Si. For instance, if a porous Si layer shows a porosity of 50%, it may be safe to assume that its mechanical strength is about a half of that of a comparable bulk Si layer. In other words, when a wafer formed by bonding a pair of substrates is subjected to compressive, tensile or shearing force, the porous Si layer arranged therebetween will be destroyed firstly. If the layer has a large porosity, it will be destroyed with little effort.

As pointed above under [Preparation of a Diffusion Region], the process of transforming a diffusion region into a porous state can easily spread beyond the diffusion region. For the purpose of the present invention, the thickness of the porous layer is generally between 1 and 150µm, preferably between 2 and 80µm and most preferably between 5 and 50µm from the viewpoint of the time required for forming the porous layer and the desired level of fragility of the porous layer.

For the purpose of the present invention, a nonporous semiconductor layer can be formed preferably by using a material selected from single crystal Si, polycrystalline Si, noncrystalline Si and compound semiconductors including GaAs, InP, GaAsP, GaAlAs, InAs, AlGaSb, InGaAs, ZnS, CdSe, CdTe and SiGe. A nonporous semiconductor layer that can be used for the purpose of the present invention may substantially contain one or more than one FETs (field effect transistors).

For the purpose of the present invention, the first substrate is a silicon substrate having therein a porous silicon layer and carrying a nonporous semiconductor layer arranged on the porous silicon layer. It may be prepared by forming a nonporous semiconductor layer on the porous silicon layer in the silicon substrate or by forming a porous silicon layer in part of a silicon substrate having therein a nonporous semiconductor substrate.

A nonporous semiconductor layer can be formed on a porous silicon layer typically by means of a CVD technique selected from vacuum CVD, plasma CVD, photo CVD and MO CVD (metal-organic CVD), a sputtering technique (including a bias sputtering technique), a molecular beam epitaxial growth technique or a liquid phase growth technique.

For the purpose of the present invention, the second substrate onto which the nonporous semiconductor layer is transferred from the first substrate may be selected from a semiconductor substrate such as a single crystal silicon substrate, a semiconductor substrate carrying an insulation film such as an oxide film (including a thermally oxidized film) or a nitride film on the surface thereof, a light transmitting substrate such as a silica glass substrate or a glass substrate, a metal substrate and an insulating substrate typically made of alumina depending on the application of the finally prepared semiconductor article.

For the purpose of the invention, the first substrate comprising a porous silicon layer and a nonporous semiconductor layer can be firmly bonded to a second substrates to produce a multilayer structure (in such a way that the nonporous semiconductor layer is located inside). For the purpose of the present invention, a multilayer structure having a nonporous semiconductor layer located inside refers not only to a structure obtained by bonding the nonporous semiconductor layer of a first substrate directly to a second substrate but also to a structure obtained by bonding the nonporous semiconductor layer of a first substrate carrying on the surface thereof an insulation film such as an oxide film, a nitride film or some other film to a second substrate. In other words, a multilayer structure having a nonporous semiconductor layer located inside refers to a structure where the nonporous semiconductor layer is located inside relative to the porous silicon layer.

For the purpose of the present invention, the first and second substrates can be firmly bonded together typically at room temperature by smoothing their bonding surfaces. Additionally, techniques including anodic bonding, pressurization and thermal treatment may appropriately be used to improve the bonding strength.

Since the porous silicon layer formed outside the diffusion region is fragile if compared with the porous silicon layer formed inside the diffusion region and hence can be easily destroyed in the process of separating the first and second substrates so that the two substrate can be separated reliably. Specific techniques that can be used for splitting the multilayer structure include application of external force that may be in the form of pressure, pulling force or shearing force, application of internal force by oxidizing and expanding the porous silicon, generation of thermal stress by applying pulsation of heat to the multilayer structure and simply softening the porous silicon layer as well as other appropriate techniques.

After separating the multilayer structure that has been prepared by bonding first and second substrates together along the porous Si layer, the residual porous Si remaining on the substrates can be selectively removed on the basis of the fact that the porous Si layer has a low mechanical strength and a large surface area. Methods that can be used for selectively removing the remaining porous Si include mechanical techniques such as scraping and polishing, chemical etching using an etching solution and ion etching (such as reactive ion etching).

Etching solutions that can be used for a process of selectively removing the porous Si by means of an etching solution include, beside a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, hydrofluoric acid, a mixture solution obtained by adding alcohol to hydrofluoric acid, a mixture solution obtained by adding alcohol and aqueous hydrogen peroxide to hydrofluoric acid, buffered hydrofluoric acid, a mixture solution obtained by adding alcohol to buffered hydrofluoric acid, a mixture solution obtained by adding aqueous hydrogen peroxide to buffered hydrofluoric acid, a mixture solution obtained by adding alcohol and aqueous hydrogen peroxide to buffered hydrofluoric acid and a mixture solution of hydrofluoric acid, nitric acid and acetic acid.

The semiconductor article having the transferred nonporous semiconductor layer may preferably be heat treated in a hydrogen-containing atmosphere, subsequently to the selective removal of the porous layer, to improve the flatness of the nonporous semiconductor layer.

Now, the present invention will be described by referring to the accompanying drawings that illustrate preferred modes of carrying out the invention.

In this mode of carrying out the invention, a P⁺ layer 12 is formed by diffusion on the principal surface of a single crystal silicon substrate 11 (FIG. 1A). Thereafter, the P⁺ layer on the principal surface is transformed into a porous state to a depth greater than the thickness of the P⁺ layer to produce a P⁺ porous silicon layer 13 and an underlying porous silicon layer 14 (FIG. 1B). Then, at least a single nonporous thin film 15 is formed on the P⁺ porous silicon layer 13. Now, a first substrate is prepared. The nonporous thin film 15 may comprise single crystal Si, polycrystalline Si, noncrystalline Si, metal film, compound semiconductor thin film or superconductive thin film. The bonding interface can be separated from the active layer preferably by additionally forming an uppermost SiO₂ layer. Referring to FIG. 1C, a surface of the second substrate 16 and a corresponding surface of the first substrate are brought into close contact so that they may be bonded together to produce a multilayer structure having the nonporous thin film 15 is located inside. Subsequently, the bonding strength between the two substrates can be increased by means of anodic bonding, pressurization, heat treatment, if appropriate, or a combination of any of these.

When a single crystal Si layer is produced by deposition, preferably silicon oxide is formed on the surface of the single crystal Si layer typically by thermal oxidation before the substrates are bonded together. The second substrate may be any of the above listed candidates. While FIG. 1C shows a second substrate bonded to a first substrate with an insulation layer 17 arranged therebetween, the insulation layer 17 may be omitted when the nonporous thin film 15 or the second substrate is not made of Si. A thin insulating panel may be arranged between the first and second substrates when bonding them together.

When the nonporous thin film is made of epitaxially grown single crystal silicon or some other similar material, the pores inside the porous Si layer can be rearranged and closed to reduce the etchability of the porous Si layer at the time of etching if it is subjected to heat treatment during the process of epitaxial growth or in a subsequent process. In order to avoid this problem and improve the structural stability of the porous layer, the porous Si layer is preferably subjected to a preliminary heat treatment operation that is conducted at temperature between 200 and 700°C to form a thin oxide film on the wall surface of the pores (while maintaining the properties of porous single crystal silicon) and prevent any possible rearrangement of the pores.

A step as will be described below can be employed to produce an epitaxial silicon film that is substantially free from defects.

While a porous Si layer maintains the structure of single crystal silicon, the epitaxial silicon film formed on the surface can show defects attributable to the numerous pores existing on the surface of the porous Si layer. Therefore, it may be a good idea to hermetically close the surface of the porous Si layer that is brought into contact with the epitaxial film by means of single crystal Si.

A technique that can be used for hermetically closing the surface of the porous Si layer is a heat treatment operation to be conducted in a hydrogen-containing atmosphere. As a result of this heat treatment using hydrogen, some of the silicon atoms on the surface of the porous Si layer will be migrated to hermetically close the pores exposed to the surface of the porous Si layer. This heat treatment operation is typically conducted at temperature between 500 and 1,300°C, preferably between 900 and 1,300°C.

Apart from this technique, it may also be effective to form a silicon film on the surface of the porous Si layer at a very low rate to close the pores exposed to the surface of the layer by allowing gas that contains silicon atoms to flow into the film forming chamber.

In the above described process of closing the pores exposed to the surface of the porous Si layer and forming a silicon film by epitaxial growth after the formation of a thin oxide film on the wall surface of the pores, the single crystal is preferably exposed at the top of the porous Si layer to effectively close the pores. The single crystal can be exposed by immersing the upper surface of the porous Si layer whose pores have been coated with thin oxide film in an acid such as HF to remove the thin oxide film arranged on the upper surface.

Thereafter, the two substrates are separated along the porous silicon layer 14 (FIG. 1D). Any of the above described techniques may be used for separating the substrates.

While the two substrates may be separated mainly along the fragile underlying porous silicon layer 14, they may partly or totally be separated through the P⁺ porous silicon layer 13 without any problem.

Thereafter, the porous Si layers 13 and 14 are selectively removed. If the nonporous thin film is made of single crystal Si, only the porous Si layers 13 and 14 are etched off by nonelectrolytic wet chemical etching by using an etching solution prepared for ordinary Si etching, hydrofluoric acid that is an etching solution for selectively etching porous Si, a mixture solution obtained by adding at least either alcohol or aqueous hydrogen peroxide to hydrofluoric acid, buffered hydrofluoric acid or a mixture solution obtained by adding at least either ethanol or aqueous hydrogen peroxide to buffered hydrofluoric acid to leave the film that has been formed on the first substrate in advance on the second substrate. As described above in detail, it is possible to selectively etch only the porous Si by means of an etching solution prepared for ordinary Si etching because of the large surface area of the porous Si layers 13 and 14. Alternatively, the porous Si layers 13 and 14 may be selectively removed by polishing it, using the nonporous thin film layer 15 as a polishing stopper.

When a compound semiconductor layer is formed on the porous Si layer, an etching solution that provides a high Si etching rate relative to the compound semiconductor is used to chemically etch only the porous Si layers 13 and 14, leaving the thin single crystal compound semiconductor film layer 15 on the second substrate 16. Alternatively, the porous Si layers 13 and 14 may be selectively removed by polishing it, using the single crystal compound semiconductor layer 15 as a polishing stopper.

FIG. 1E shows a semiconductor article that can be produced by a method according to the invention. A large nonporous thin film which is typically a single crystal Si thin film 15 is evenly and thinly formed on the entire surface of the second substrate 16. If an insulating substrate is used for the second substrate 16, the prepared semiconductor substrate can advantageously be used for producing electronic devices that are insulated and separated from each other.

Once the residual porous Si on the first single crystal Si substrate 11 is removed from the latter, the latter can be used as another first single crystal Si substrate 11 or as another second substrate 16 after smoothing the surface if the surface has turned impermissibly coarse and such a smoothing operation is necessary.

FIGS. 2A through 2E illustrate a second mode of carrying out the invention. As shown, a P⁺ layer 22 is formed on each of the oppositely disposed surfaces of a single crystal silicon substrate 21 that constitutes a first substrate by means of a diffusion technique (FIG. 2A). Then, the first substrate is transformed into a porous state on the both sides to a depth greater than the thickness of the P⁺ layers 22 to produce P⁺ porous layers 23 and an underlying porous silicon layer 24 (FIG. 2B). Subsequently, a nonporous thin film 25 is formed on each P⁺ porous layer 23 and then second substrates 26 and 27 are bonded to the respective sides with an insulation layer 28 disposed therebetween (FIG. 2C). Thus, it will be appreciated that a pair of semiconductor articles are prepared in a single process. Otherwise, the manufacturing steps of this mode are identical with those of the above described first mode.

Once the residual porous Si on the single crystal Si substrate 21 is removed from the latter, the latter can be used as another first single crystal Si substrate 21 or as another second substrate 26 (or 27) after smoothing the surface if the surface has turned impermissibly coarse and such a smoothing operation is necessary.

The support substrates 26, 27 may have respective thicknesses that are different from each other. The nonporous thin films 25 on the opposite surfaces of the first substrate may be made of respective materials and have respective thicknesses that are different from each other.

Now, the present invention will be described further by way of examples.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique. Another P⁺ high concentration layer was also formed on the rear surface of the substrate.

More specifically, the P⁺ high concentration layer was formed in the following manner. A solution prepared by dissolving B₂O₃ into a solvent was applied to the principal surface of an Si substrate by means of a spin-coating technique, which was then baked at 140°C to dissipate the solvent. The resultant substrate was then placed in a diffusion furnace and subjected to a so-called driven-in diffusion process to produce a P⁺ high concentration layer, maintaining the temperature of the furnace tube to 1,200°C for six hours. The spin-coated film was removed.

Subsequently, the Si substrate carrying thereon a P⁺ high concentration layer was immersed in an HF solution to carry out an anodization process from the first surface side of the substrate and produce a porous layer on the first surface side. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴ Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were brought into contact with each other and put together to produce a multilayer structure. The 100nm thick SiO₂ layer and the epitaxial Si layer were removed by etching from an edge of the bonded wafer to expose an edge of the porous silicon layer.

Subsequently, the bonded wafer was subjected to pyro-oxidation at 1,000°C to completely separate the two substrate along the lower porous silicon layer within 50 minutes. The separated surfaces were observed to find out that the porous silicon had been transformed into SiO₂ on the outer periphery of the wafer, while it remained unaffected in a central area.

Thereafter, the porous Si layer and the oxidized porous Si layer remaining on the second substrate were selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si and the oxidized porous Si were selectively etched and removed completely.

The rate of etching nonporous single crystal Si by means of the above cited etching solution is very low and the selectivity ratio of the rate of etching porous Si relative to that of etching nonporous single crystal Si is as large as more than 10⁵ so that the reduction by etching of the height of the nonporous layer (about tens of several angstroms) is practically negligible.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the root mean square of surface roughness (Rrms) within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si and the oxidized porous silicon remaining on the first substrate were also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si and the oxidized porous Si were selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 1. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were brought into contact with each other and put together.

After removing the oxide film on the rear surface of the Si substrate, a CO₂ laser beam was irradiated on the entire Si substrate side surface of the wafer with an output power level of 500 to 1,000W. The CO₂ laser was absorbed by the 500nm thick SiO₂ layer arranged on the interface of the two substrates to rapidly raise the temperature of the epitaxial layer and the porous Si layer that were located close to it until the two substrates were separated from each other along the underlying porous Si layer due to the thermal stress rapidly generated in the underlying porous Si layer.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique. Another P⁺ high concentration layer was also formed on the rear surface of the substrate.

More specifically, the P⁺ high concentration layer was formed by means of a diffusion technique in the following manner. An Si substrate was placed in a diffusion furnace and then N₂ gas was introduced into a liquid diffusion source containing BBr₃ for bubbling. The produced gas was then introduced into the furnace tube with a mixture carrier gas of (N₂+O₂). A B₂O₃ layer was formed by maintaining the temperature of the furnace tube to 1,050°C for an hour and subsequently the substrate was subjected to a so-called driven-in diffusion process to produce a P⁺ high concentration layer, maintaining the temperature of the furnace tube to 1,200°C for six hours.

Then, the high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂ / H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1g10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were brought into contact with each other and put together.

A pulsated electric current of about 10 to 100A was made to flow only through the high concentration P⁺ single crystal Si layer of the first substrate. The electric current was made to flow by removing the SiO₂ to expose the high concentration P⁺ single crystal Si layer at an end surface of the wafer and pinching the wafer by means of + and - electrodes that touch only the end surface. As a result, the underlying porous Si layer was abruptly subjected to thermal stress to sever the two substrates along the underlying porous Si layer.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 5µm thick P⁺ high concentration first layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 3. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were exposed to nitrogen plasma (in order to improve the bonding strength) and then laid one on the other to bring them into contact with each other. The combined substrates were then annealed at 400°C for 10 hours.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si . layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 1. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were exposed to nitrogen plasma (in order to improve the bonding strength) and then laid one on the other to bring them into contact with each other. The combined substrates were then annealed at 400°C for 10 hours.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Then, the porous Si remaining on the second substrate was also selectively and completely eteched in an HF/HNO₃/CH₃COOH type etching solution. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in an etching solution of HF/HNO₃/CH₃COOH type, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 1.

Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of a quartz substrate (second substrate) prepared in advance were exposed to nitrogen plasma and then put together to bring them into contact with each other. The combined substrates were then annealed at 200°C for 10 hours.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the quartz substrate. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 970°C for two hours in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate.

A 5µm thick P⁺ high concentration first layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 3. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon.

Single crystal Si was made to epitaxially grow to a thickness of 0.55µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were brought into contact with each other and put together. Thereafter, the 100nm thick SiO₂ layer and the epitaxial Si layer were removed by etching from an edge of the bonded wafer to exposes an end of the porous silicon layer.

Subsequently, the bonded wafer was subjected to pyro-oxidation at 1,000°C to completely separate the two substrate along the lower porous silicon layer within 50 minutes. The separated surfaces were observed to find out that the porous silicon had been transformed into SiO₂ on the outer periphery of the wafer, while it remained unaffected in a central area.

Thereafter, the porous Si layer and the oxidized porous Si layer remaining on the second substrate were selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si and the oxidized porous Si were selectively etched and removed completely.

The rate of etching nonporous single crystal Si by means of the above cited etching solution is very low and the selectivity ratio of the rate of etching porous Si relative to that of etching nonporous single crystal Si is as large as more than 10⁵ so that the reduction by etching of the height of the nonporous layer (about tens of several angstroms) is practically negligible.

Thus, a single crystal Si layer was formed to a thickness of 0.5µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 500nm±15nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si and the oxidized porous silicon remaining on the first substrate were also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si and the oxidized porous Si were selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate of a regenerated wafer by means of a diffusion technique as in the case of Example 3. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were brought into contact with each other and put together.

After removing the oxide film on the rear surface of the Si substrate, a CO₂ laser beam was irradiated on the entire Si substrate side surface of the wafer with an output power level of 500 to 1,000W. The CO₂ laser was absorbed by the 500nm thick SiO₂ layer arranged on the interface of the two substrates to rapidly raise the temperature of the epitaxial layer and the porous Si layer that were located close to it until the two substrates were separated from each other along the underlying porous Si layer due to the thermal stress rapidly generated in the underlying porous Si layer.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 5µm thick P⁺ high concentration first layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 1. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon.

Single crystal GaAs was made to epitaxially grow to a thickness of 1µm on the porous Si layer by means of a MOCVD (metal organic chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas TMG/AsH₃/H₂ gas pressure 1.1x10⁴Pa (80 Torr) temperature 700°C

The surface of the GaAs layer and the corresponding surface of an Si substrate (second substrate) prepared in advance were brought into contact with each other and put together.

A pulsated electric current of about 10 to 100A was made to flow only through the high concentration P⁺ single crystal Si layer of the first substrate. The electric current was made to flow by removing the SiO₂ to expose the high concentration P⁺ single crystal Si layer at an end surface of the wafer and pinching the wafer by means of + and - electrodes that touch only the end surface. As a result, the underlying porous Si layer was abruptly subjected to thermal stress to sever the two substrates along the underlying porous Si layer.

Thereafter, the residual porous Si layer on the second substrate was etched off by means of:
   ethylenediamine + pyrocatechol + water (at a ratio of 17ml : 3g : 8ml) at 110°C.

The single crystal GaAs was left unetched and operated as etching stopper so that the porous Si and the oxidized porous Si was selectively etched and removed completely.

The rate of etching nonporous single crystal GaAs by means of the above cited etching solution is very low and the reduction by etching of the height of the nonporous layer (about tens of several angstroms) is practically negligible.

Thus, a single crystal GaAs layer was formed to a thickness of 1µm on the silicon oxide film. The thickness of the formed single crystal GaAs layer was observed at 100 points spreading over the entire surface of the substrate to find that the degree of uniformity of the film thickness was 1µm±29.8nm.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the GaAs layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a GaAs layer was formed on an insulation film by using an Si substrate carrying thereon an oxide film as support substrate to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another bonding process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 1.
Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 (min.)
   thickness of the porous Si layer: 12 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film. Single crystal InP was made to epitaxially grow to a thickness of 1µm on the porous Si layer by means of a MOCVD (metal organic chemical vapor deposition) technique.

The surface of the InP layer and that of a quartz substrate (second substrate) prepared in advance were brought into contact with each other and put together. The combined substrates were annealed at 200°C for 10 hours.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal InP was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal InP layer was formed to a thickness of 1µm on the silicon oxide film. The thickness of the formed single crystal InP layer was observed at 100 points spreading over the entire surface of the substrate to find that the degree of uniformity of the film thickness was 1µm±29.0nm.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the InP layer and an excellent degree of crystallinity had been maintained.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate.

A 5µm thick P⁺ high concentration first layer was formed as a surface layer on the two opposite sides of a double mirror single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 1. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution from the rear surface side. The anodization was conducted under the following conditions. The anodizating operation was conducted on one of the surfaces at a time for 11 minutes.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 11 × 2 (min.)
   thickness of the porous Si layer: 12 (µm)

Each of the obtained porous silicon layers had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on each of the porous Si layers by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of each of the epitaxially grown Si layers.

The surface of each of the SiO₂ layers and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were brought into contact with each other and put together so that the first substrate was sandwiched by a pair of second substrates. Thereafter, the 100nm thick SiO₂ layer and the epitaxial Si layer were removed by etching from an edge of the bonded wafer to exposes an end of the porous silicon layer.

Subsequently, the bonded wafer was subjected to pyro-oxidation at 1,000°C to completely separate the two substrate along the lower porous silicon layer within 50 minutes. The oxidation process is preferably conducted for several hours in order to minimize any deviations among wafers. The separated surfaces were observed to find out that the porous silicon had been transformed into SiO₂ on the outer periphery of the wafer, while it remained unaffected in a central area.

Thereafter, the porous Si layers and the oxidized porous Si layers remaining on the two second substrates were selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si and the oxidized porous Si were selectively etched and removed completely.

Thus, a pair of 0.1µm thick single crystal Si layers were formed on the silicon oxide film. The thickness of each of the formed single crystal Si layers was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si and the oxidized porous silicon remaining on the first substrate were also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si and the oxidized porous Si were selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 10µm thick P⁺ high concentration first layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 1 except that the drive-in diffusion process was conducted for 10 hours. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 15 (min.)
   thickness of the porous Si layer: 16 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) prepared in advance were brought into contact with each other and put together. Thereafter, the wafer was heat treated at 1,180°C for five minutes to improve the bonding strength.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 10µm thick P⁺ high concentration first layer was formed as a surface layer of a single crystal silicon substrate having an unspecified electric resistance by means of a diffusion technique as in the case of Example 1 except that the drive-in diffusion process was conducted for 10 hours. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 15 (min.)
   thickness of the porous Si layer: 16 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were brought into contact with each other and put together. Thereafter, the wafer was heat treated at 1,180°C for five minutes to improve the bonding strength.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another bonding process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate of a regenerated wafer by means of a diffusion technique. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 15 (min.)
   thickness of the porous Si layer: 16 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressur 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) carrying a 500nm thick SiO₂ layer thereon and prepared in advance were exposed to nitrogen plasma (in order to improve the bonding strength) and then laid one on the other to bring them into contact with each other. The combined substrates were then annealed at 400°C for 10 hours.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

For the purpose of comparison, a similar multilayer structure carrying no oxide film on the epitaxial Si layer was prepared to obtain the above identified results.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another oxide film forming process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate of a regenerated wafer by means of a diffusion technique as in the case of Example 1. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 15 (min.)
   thickness of the porous Si layer: 16 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon. Single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) prepared in advance were exposed to nitrogen plasma (in order to improve the bonding strength) and then laid one on the other to bring them into contact with each other. The combined substrates were then annealed at 400°C for 10 hours.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another bonding process as a second substrate.

A 5µm thick P⁺ high concentration layer was formed as a surface layer of a single crystal silicon substrate of a regenerated wafer by means of a diffusion technique as in the case of Example 1. Another P⁺ high concentration layer was also formed on the rear surface of the substrate. The high concentration surface layer of the single crystal Si substrate was subjected to anodization in an HF solution. The anodization was conducted under the following conditions.
   current density: 7 (mA•cm^-2)
   anodization solution: HF : H₂O : C₂H₅OH = 1 : 1 : 1 duration: 15 (min.)
   thickness of the porous Si layer: 16 (µm)

The obtained porous silicon layer had a double-layered structure, of which the underlying lower porous silicon layer showed a minute and fragile structure if compared with the upper surface layer.

The substrate was then oxidized at 400°C for an hour in an oxygen atmosphere. As a result of the oxidation, the wall surfaces of the pores of the porous Si were covered with thermally oxidized film of silicon.

Thereafter, the thin oxide film produced on the uppermost surface of the substrate where the porous layer had been formed was removed by immersing it in a 1.25% HF solution. Subsequently, the obtained substrate was subjected to heat treatment at 1,050°C and 1.0x10⁵Pa (760 Torr) for 1 minute in a flow of H₂ flowing at a rate of 2301/min. and for more 5 minutes after adding SiH₄ by 50sccm.

Then, single crystal Si was made to epitaxially grow to a thickness of 0.15µm on the porous Si layer by means of a CVD (chemical vapor deposition) technique. This operation was conducted under the following conditions. source gas SiH₂Cl₂/H₂ gas flow rate 0.5/180 l/min. gas pressure 1.1x10⁴Pa (80 Torr) temperature 950 °C growth rate 0.3 µm/min.

Additionally, an SiO₂ layer was formed to a thickness of 100nm by thermally oxidizing the surface of the epitaxially grown Si layer.

The surface of the SiO₂ layer and the corresponding surface of an Si substrate (second substrate) prepared in advance were exposed to nitrogen plasma (in order to improve the bonding strength) and then laid one on the other to bring them into contact with each other. The combined substrates were then annealed at 400°C for 10 hours.

When evenly distributed and sufficiently strong tensile force was applied onto the entire surface of the bonded wafer in a direction perpendicular to the surface, the substrates were separated along the underlying lower porous silicon layer that was very fragile. A similar result was obtained when pressure or shearing force was used in place of the tensile force. The substrates could also be separated from each other along the fragile lower porous silicon layer by driving a knife into a gap between the bonded two substrates.

Thereafter, the residual porous Si layer on the second substrate was selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely.

Thus, a single crystal Si layer was formed to a thickness of 0.1µm on the silicon oxide film. The thickness of the formed single crystal Si layer was observed at 100 points spreading over the entire surface of the substrate to find that the uniformity of the film thickness was 101nm±3nm.

Then, the substrate was subjected to a heat treatment operation at 1,100°C for an hour in a hydrogen atmosphere. The surface coarseness was observed by means of atomic force microscopy to find that the Rrms within a 50µm square was about 0.2nm, which is substantially equal to the corresponding value of commercially available Si wafers.

When a cross section was observed through a transmission electron microscope, it was confirmed that no new crystal defects had been introduced in the Si layer and an excellent degree of crystallinity had been maintained.

Finally, the porous Si remaining on the first substrate was also selectively etched off in a mixture solution of 49% hydrofluoric acid and 30% aqueous hydrogen peroxide, stirring the solution constantly. The single crystal Si was left unetched and operated as etching stopper so that the porous Si was selectively etched and removed completely and the first substrate could be used for another diffusion process of the high concentration P⁺ layer as another first substrate or for another bonding process as a second substrate.